Buyout assignments that are on sale?

I wanted to get this product (that was my first attempt at linking... so sorry if it didn't work right) for a 30% buyout assignment. Right now, the product is $18.84 if you just buy it regular from the site, because it's on sale. When I try to get it as an assignment, and do the 30% buyout, it tells me it's $20.29. Do you not get the 30% off of the current price of the item? Or is this a glitch?